Sidhu was administered the oath of office and secrecy by Punjab Governor V.P. Singh Badnore.
However, Sidhu has not been designated as Deputy Chief Minister, as was being speculated in political circles.
Sidhu, who joined the Congress party in January, just days ahead of the February 4 assembly elections in Punjab, won the Amritsar-east assembly seat with a margin of over 42,000 votes.
Sidhu had earlier remained Member of Parliament (MP) from Amritsar when he was part of the Bharatiya Janata Party (BJP). He was elected MP in 2004, 2007 (by-election) and 2009.
He was nominated to the Rajya Sabha by the Narendra Modi government in April 2016.